A commonly used method for applying a coating to a paper web in a paper machine is by using some type of roll applicator. A size press is one example using roll applicators. Heretofore, when coating in a size press, a more or less pronounced "orange peel" effect occurred, due to an adhesion force created between the coating mixture, the rolls, and the web, such that a part of the mixture often followed the rolls after the nip. This meant that the mixture split in two directions; i.e., the paper web and the rolls. That is, a short distance after the nip, the mixture "broke" and was sucked towards the paper and towards the roll, thereby creating minute spots of mixture which gave the paper the appearance of an orange peel, thereby resulting in an unsatisfactory paper product. A general object of the present invention is to provide a solution to the aforesaid problem.
Coatings used for such paper products can be roughly divided into two groups; namely pigment and functional. Pigment coating is the most common type and a mixture thereof comprises at least one pigment, for example, clay calcium carbonate, satin white or titanium dioxide. The main purposes of this kind of coating are: (1) to achieve an even surface which is favorable for printing color; (2) to obtain a better opacity in the final paper; and (3) to mask or change the color of the base paper.
Functional coating consists of coating or impregnating the paper in such a way that a special function is obtained. Properties which make the paper water repellent, grease resistant or impenetrable to water vapor are examples of functional coatings. In the examples mentioned above, it is important that the coating layer consists of a substantially even film, i.e., one of uniform depth or thickness; this is especially important when a water vapor barrier is needed. Also, analogous to pigment coating, no orange peel effect must occur. In a similar manner, this effect can be decreased or eliminated by adding small amounts of the bifunctional composition described below to the mixture. Another big problem is that the runability is very limited due to deposits on the rolls. In many cases, some kind of latices make the make the whole or most of the active part of the mixture. Examples of these are bases of styrene butadiene, acrylic acid or polyvinylidene chloride (PVDC). These, and especially PVDC, have an inherent tendency to stick to the rolls, thereby causing uneven coating and web breakage and this limits the running time because of the need for cleaning rolls.
A further object of the present invention is to provide an additive material for paper coatings which will reduce the aforesaid problems by enabling the machine rolls to be kept clean and thereby eliminate web breaks during continuous production.
During the manufacture of paper, another type of coating can be applied to obtain higher surface strength, decreased linting and higher stiffness. This type of coating is a solution of starch added either at the size press or at the calender stack. In calender sizing, the concentration of starch must be kept low to prevent web adhesion to the steel rolls and formation of strings and scabs that mark the web. At the size press, a high concentration of starch cannot be used because the web will adhere to the rolls and cause the web to break. Also the web may stick to the following dryer cans (also called drums or cylinders) or leave a starch coating on them that reduces drying efficiency. Thus, another object of the invention is to provide a bifunctional composition of the type mentioned above that will decrease adhesion of the paper web to the rolls and dryers while enabling the starch concentration to be increased. When the starch concentration is increased at the size press, less water is needed to apply the same amount of dry starch per square meter of paper, and this allows increased machine speed or the addition of more starch to the web.